As 2014 has worn on — and 2016 has drawn closer — one thing has gone from generally agreed upon to rock-solid conventional wisdom: If Hillary Clinton runs for the Democratic presidential nomination, she wins the Democratic presidential nomination.

Former secretary of state Hillary Clinton waits to speak at the World Bank on May 14  in Washington, D.C. Clinton and World Bank President Jim Yong Kim joined others to speak about women's rights. (Brendan Smialowski/AFP/Getty Images)

No one has emerged — as then-Illinois Sen. Barack Obama had by this point in the 2008 election — who, even in theory, has an obvious (or not-so-obvious) path to upsetting Clinton. And, as we have written before, Clinton starts in a far stronger position than she occupied in 2008. Now, of course, things could change. A candidate no one is talking about (or even thinking about) right now could catch fire. And, as Aaron Blake points out, her numbers will start dropping the second she becomes a candidate. But the reality is that if nothing extraordinary happens, the only person who could keep Clinton from being the party’s nominee would be Clinton herself.

Although we — and virtually everyone else in the political world — have begun to take her candidacy as a foregone conclusion, no one in her real inner circle is talking — meaning that she almost certainly hasn’t made the final go/no-go decision. So, what could keep her from running? For all the sturm und drang caused by Karl Rove's insinuations about Clinton's health scare in late 2012, a blood clot in the brain is a serious thing, and even her allies acknowledge that it was not insignificant. Bill Clinton insisted this week that his wife is in "better shape" than he is, and her aides say she has an absolutely clean bill of health. But Clinton would be 69 years old on Election Day 2016. Couple her age with that scare in late 2012 and it's hard to imagine Clinton not taking a full accounting of her health before officially deciding to run.

Possible that such an accounting leads Clinton to take a pass? Yes. Likely? No. Below are our rankings of the candidates most likely to run for and/or win the Democratic nomination in 2016.  We've split them into tiers to accurately reflect the state of play at the moment.

First tier (The Clinton Wing)

* Hillary Clinton: This requires no further explanation.

Second tier (Wait, she's not running?!?!?!)

* Joe Biden: The vice president spoke to a gathering of Des Moines business leaders during their trip to D.C. this month. He also went to South Carolina to raise money for the state party that week. In case you just landed from another planet, that means he wants to run for president. But senior Democratic insiders agree with the sentiment we have expressed in this space before: Biden knows that if Clinton runs, he has a very steep hill to climb.

Martin O'Malley: O'Malley, term limited out as governor of Maryland at the end of this year, is running — and running hard — for president. A recent Politico story reported that O'Malley had gotten the go-ahead from Clinton to pursue his 2016 ambitions while she makes up her mind. He's doing just that, but it may all be for naught; if Clinton runs, it's going to be hard for O'Malley to gain much/any traction against her.

* Elizabeth Warren: The senator from Massachusetts is the anti-corporatist liberal icon who  could draw the right contrasts with Clinton to make a race out of the primary. But she's not running against Clinton. If Clinton gets out, If Clinton gets out, Warren will probably be the front-runner, along with Biden. But questions remain about whether Warren could grow beyond her admittedly ardent base of liberal supporters.

Third tier (So you're telling me there's a chance ...)

* Andrew Cuomo: The New York governor's 2016 stock has slipped steadily over the past year or so.  What many people once saw as Cuomo playing possum to avoid the national spotlight is now being read as a genuine disinterest in running for the nation's highest office. And, even those who think Cuomo might run in a Hillary-less field raise questions about the insularity of his political team and his ties to Wall Street.

Kirsten Gillibrand: If Clinton doesn't run, we are convinced that the senator from New York would be well positioned to be a serious contender for the nomination. Her work on sexual assault in the military has raised her profile in a major way among Democratic activists, and, as her past races make clear, she can raise money like very few others in the party. Outside of Clinton, she may have the most buzz among party-strategist types.

* Deval Patrick: The Massachusetts governor has gone quiet after opening the door to at least thinking about a 2016 race earlier this spring. Patrick is a gifted speaker who would probably play well among activists on the stump. And, if he was the lone African American candidate in a Hillary-less field, Patrick would have a major leg up.

* Brian Schweitzer: The former Montana governor wants to run for president — like crazy. And for anyone who has seen Schweitzer speak or spent more than 30 seconds in his presence, his charisma is apparent. Running as a sort of prairie populist in a place like Iowa could make Schweitzer interesting, but there are real questions about whether his schtick would wear thin over the course of a long primary process.

Fourth tier (Probably not ... but maybe)

* John Hickenlooper: After a strange dip in his poll numbers earlier this year, the Colorado governor looks to be in solid shape to win a second term.  If he does, expect the 2016 talk that bubbled up before his 2012 divorce separation to come back. What's hard to know is whether Hickenlooper has the sort of unrelenting ambition to run for national office in the event Clinton says no.

* Amy Klobuchar: The senator from Minnesota is ambitious, and we'd bet that she runs for president sometime before her political career is over. The 2016 race is probably not that time, though.

* Jay Nixon: Two-time elected governor of a Republican-leaning state in the middle of the country? Not a bad résumé on which to run an insurgent campaign in a field without Clinton. And, Nixon was decidedly coy in February when asked about whether 2016 interested him.

* Mark Warner: We're still slightly baffled as to why Warner pulled the plug on what seemed like a certain bid for president in 2008. As the most popular politician in a swing state, Warner would be taken seriously if he ran in 2016. But can someone who is a self-avowed moderate really make a serious bid at the Democratic presidential nomination?